1. Field of the Invention
The present invention relates generally to a disc brake as a braking device for automotive vehicles and motorcycles, and more particularly to an improvement in a rotor or disc of the disc brakes.
2. Description of the Prior Art
For example, in the event that vehicles such as automotive vehicles and motorcycles are exported to a foreign country via a sea route, the vehicles assembled in a factory are first stored in a storage place near a loading port and next loaded onto a transport ship which travels to the foreign country which imports the vehicles. When they reach the foreign country, the vehicles are again stored in a storage place near an unloading port and thereafter are handed over to customers or users.
However, in the case where disc brakes are employed in the above-mentioned vehicles, the rotor of the disc brakes may be rusted for the following reasons: (1) The disc brake rotor is usually made of a ferrous metal; (2) The air of the storage place near the loading port contains much salt; (3) The inside of the transport ship is high in temperature and humidity; and (4) The total of the storage term in the storage places near the loading and unloading ports and the voyage term to the foreign country may reach several months due to unexpected external factors. When the disc brake rotor is rusted, the degree of rusted state is different between a part of the disc brake rotor to which brake linings are brought into contact and the other part of the same. If the disc brake is operated upon the vehicle running under such a rusted condition, torque fluctuation of the disc brake rotor unavoidably occurs. This is called the `judder` phenomena and impedes a driver's smooth braking feeling.
In view of the above, it has already been proposed to use rust preventing treatment on a disc brake rotor, in which a corrosion resistance coating such as phosphate is formed on the surface of the disc brake rotor. This is effective for improving corrosion resistance and for decreasing the torque fluctuation of the disc brake rotor. However, such a phosphate coating is relatively soft to obtain the initial concordance between the disc brake rotor and the brake lining in order to secure the coefficient of friction .mu. therebetween, thus weakening the adhesion of the coating to the surface of the disc brake rotor. Additionally, increasing the thickness of the coating to improve the corrosion resistance lowers the coefficient of friction. In this regard, the thickness of the coating must be as thin as possible within a range of attaining a required corrosion resistance. This results in the possibility that the soft coating is scraped off by a brake lining when a strong brake force is applied, particularly in the case where a so-called semi-metallic brake lining (whose base material is steel fiber) is used in the disc brake. This lowers the corrosion resistance of the disc brake rotor and increases the torque fluctuation of the disc brake rotor during braking of the vehicle. The semi-metallic brake lining is higher in grinding force and recently has become widely used.